It was not well understood what might trigger PYY to send the message to stop eating.
Researchers are reporting in the September issue of Cell Metabolism, a journal published by Cell Press, that eating a high protein diet may be enough to do the trick.
"We've now found that increasing the protein content of the diet augments the body's own PYY, helping to reduce hunger and aid weight loss," said Medical Research Council clinician scientist Rachel Batterham of University College London, who led the new study.
The study fills the gap between the knowledge that people who eat high-protein diets tend to consume fewer calories, and the reason that may be so.
The researchers show in this study that eating a high protein diet naturally stimulates the body's production of PYY more than either a high-fat or high-carbohydrate diet, resulting in less hunger and consequently more weight loss.
Earlier studies at the Imperial College Faculty of Medicine, Hammersmith Campus, London showed that in humans, peripheral infusion of PYY, at a dose which produced normal postprandial concentrations, significantly decreased appetite and reduced food intake by 33% over 24 hours.
Scientists say that these finding suggest that PYY physiologically regulates food intake.
In other words PYY is the body's natural appetite switch.
When PYY gets turned on the brain says to stop eating.
The University College researchers found that genetically modified mice unable to produce PYY ate more and became manifestly obese.
The genetically modified mice were also resistant to the beneficial effects of a high-protein diet, demonstrating a direct connection between protein and PYY.
When the researchers treated the hormone-deficient mice with PYY, they lost weight.
"The findings show that PYY deficiency can cause obesity and that PYY appears to mediate the beneficial effects of increased-protein content diets," Batterham said.
"One potential weight loss strategy is therefore to increase the satiating power of the diet and promote weight loss through the addition of dietary protein--harnessing our own satiety system.
"Such a diet is perhaps more typical to that of our hunter-gatherer ancestors," she added.
The average Western diet derives 49% of energy intake from carbohydrate, 35% from fat, and 16% from protein, Batterham said.
Hunter-gatherers likely ate as much as twice the amount of protein as their modern counterparts.
The researchers are not recommending any particular commercial diet or food plan.
They are scientists after all and would not do so without scads of documented evidence from clinical trials.
However, the scientific research that has been done suggests to this caveman that the high-protein, low carbohydrate diet plans we already have available are on target for healthy weight loss.